1. Field of the Invention
The present invention relates to a resin-coated steel sheet, more particularly, to a resin-coated steel sheet having good electrocoatability and weldability which is suitable for automotive bodies and household electric appliances.
2. Description of the Prior Art
There are increasing requirements for more corrosion resistance than before in automotive bodies and household electric appliances. These requirements are met by replacing conventional cold-rolled steel sheets and plated steel sheets with surface-treated steel sheets having much better corrosion resistance.
There are known several kinds of surface-treated steel sheets, which include zinc plated steel sheets, zinc alloy-plated steel sheets (said zinc alloy containing one or more elements such as Ni, Fe, Mn, Mo, Co, and Al), multilayer-plated steel sheets, and resin-coated steel sheets formed by coating zinc-plated or zinc alloy-plated steel sheets with a chromate layer and resin film. These conventional resin-coated steel sheets, however, still lack satisfactory electrocoatability, corrosion resistance, and spot-weldability.
There is disclosed a zinc-rich coated steel sheet in Japanese Patent Publication Nos. 24230/1970 and 6882/1972. It is receptive to electrodeposition but is poor in corrosion resistance, press formability, and weldability. There is disclosed a resin-coated steel sheet, with the resin containing an electrically conductive inorganic pigment, in Japanese Patent Publication No. 44569/1977 and Japanese Patent Laid-open Nos. 138758/1983 and 79138/1976. The resin coating film on a zinc-plated steel sheet improves corrosion resistance and weldability but suffers from a disadvantage that the electrocoating film formed thereon is poor in surface smoothness because the inorganic pigment contains comparatively coarse particles.
There is proposed an organic composite silicate thin film (0.3-3 .mu.m thick) free of electrically conductive inorganic pigments, in Japanese Patent Laid-open Nos. 224174/1983, 33192/1985, and 174879/1985. It is receptive to electrodeposition and exhibits improved corrosion resistance, weldability, and press formability; however, it does not permit a smooth electrocoating film to be formed thereon.